The proposed research will determine the structural biology of cytochrome c reductase from the mitochondrial respiratory cycle by single crystal X-ray diffraction studies. Cytochrome c reductase is an oligomeric inner mitochondrial membrane protein complex which couples the transfer of electrons from ubiquinol to cytochrome c with the translocation of protons across the membrane. This middle segment of the respiratory cycle is extremely important and many mitochondrial myopathies result from defects of this protein complex. The three dimensional structure of cytochrome c reductase, with and without inhibitors bound, would facilitate the elucidation of its mechanism of action and thereby provide initial understanding for its function and defect correlation. Even though a partial structure of this complex from a tetragonal crystal form was published less than a year ago, two of its three crucial reactive centers still remain unresolved and elusive. In an effort to resolve the complete structure with these crucial missing reactive centers intact, we have obtained different hexagonal crystal forms of the bc1 complex. Recently, native data sets to 3 Angstrom units were obtained for these hexagonal forms and the phase problem has been solved using multiple isomorphous replacement methods. The model building and refinement stages for these unique hexagonal morphologies are planned for the immediate future. Furthermore, additional data will be collected extending to the current maximum resolution to 2.5 Angstrom units utilizing improvements in flash-cooling methodology and synchrotron source and detector technologies. Inhibitor bound crystals will also be studied in an effort to deduce how different conformations are related with the functional mechanism of the bc1 complex.